THE FLINT RIVER BASIN: TECHNICAL SUMMARY OF HYDROGEOLOGY, FARM WATER USE, AND ECOLOGY The Flint River starts at Hartsfield-Jackson International Airport in Atlanta, GA. It flows southward approximately 349 miles to where it joins the Chattahoochee River to form the Apalachicola River. The Flint River drains an area of 8460 square miles, entirely within the boundaries of Georgia. The Flint has only two major impoundments on it: Lake Blackshear, near Warwick and Lake Worth, near Albany. Otherwise, the Flint River has one of the longest stretches of un-impounded flow of any river east of the Mississippi. This report provides brief technical summaries of some of the major scientific aspects of the Flint River Basin: its hydrogeology, agricultural water use in the lower Flint River Basin, and fresh-water mussel ecology. These are the principal scientific areas that will be central to the Flint River Regional Water Development and Conservation Plan. General basin description The upper half of the Flint River, from Atlanta to Highway 137 between Crawford and Taylor Counties, is within the Piedmont Province of the Appalachian Mountains. Topography ranges from rolling to mountainous. Maximum relief in the FRB is approximately 1200 ft where the Flint River flows through the Pine Mountain trend in Meriwether County. Otherwise, relief is typically less than 500 feet. The Piedmont section of the Flint River is characterized by straight segments reflecting the influence of crystalline bedrock structures such as faults, fractures and compositional trends. Furthermore, since bedrock structural trends are almost perpendicular to the river, there are numerous rocky shoals that are of great aesthetic and aquatic habitat value. In this part of the basin, the Flint River and its tributaries receive water from surface runoff, storage in saprolite, and bedrock fractures. The Flint River is almost always a gaining stream in this area. Where Highway 137 crosses the Flint River, the river crosses the Fall Line, which separates the Piedmont from the Coastal Plain Province in Georgia. At the Fall Line, the gradient of the Flint River decreases abruptly and the river’s floodplain widens as the Flint flows over coastal plain sediments instead of crystalline bedrock. South of the Fall Line, the channel pattern of the Flint switches from straight to meandering as its gradient drops and it flows over softer sediments of the Coastal Plain. A few miles north of where Turkey Creek joins the Flint River in Dooly County, the Flint enters a physiographic sub-province known as the Dougherty Plain. The Dougherty Plain is underlain by the Ocala Limestone and a heavily weathered residuum. The Ocala Limestone comprises the Floridan aquifer in the Dougherty plain region, and the aquifer is unconfined to semi-confined from Turkey Creek to the confluence of the Flint and Chattahoochee rivers. In this area, the Flint River has incised into the limestone, and thus changes channel morphology significantly. Specifically, it again becomes characterized by straight segments and what appear to be entrenched meanders as it cuts down into the Ocala Limestone. Southward from Turkey Creek, the river receives hundreds of millions of gallons of groundwater discharge from the Floridan aquifer. This stream-aquifer relationship is unique in Georgia, and is the reason that the Flint River is of such concern to resource planners and southwest Georgia stakeholders. Flint River Basin: Technical Summary of Hydrogeology, Farm Water Use, and Ecology Page 2 At the common boundary of Sumter, Crisp, and Lee counties, the Flint River makes a broad curve to the west, and flows southwestward towards Lake Seminole. Below this point, the FRB begins to exhibit an interesting asymmetry in the length of its major tributaries. In the northern half of the basin, tributaries on either side of the Flint are of approximately equal length; i.e., the Flint River flows approximately down the middle of its basin. South of the westward curve, tributaries on the west or north side of the River are far longer than those on the southern or eastern side. The latter flow directly off of the Pelham Escarpment, which is a ridge of the more resistant Tampa Limestone that overlies the Ocala. The longer tributaries drain the rest of the Dougherty Plain and the adjacent parts of the Fall Line Hills that adjoin the Dougherty Plain on the north. Hydrogeology An aquifer is a geologic formation that can store and transmit significant quantities of water. Four major aquifers underlie the lower half of the Flint River Basin. Each aquifer is separated from the ones above and be low by layers of clay or silt that impede the vertical flow of water from one aquifer to another. The aquifers are tilted to the southeast and overlie one another, such that the oldest and lowest layer is exposed farthest to the north along the Fall Line. The youngest and shallowest layer is exposed farthest to the south. The aquifers receive most of their water from the areas where they are exposed at or near land surface. These are called recharge areas. The aquifers also receive recharge as slow seepage through the clay and silt layers that confine them above and below. This process is called leakage, or leakance, by hydrogeologists. The deepest of these aquifers is the Cretaceous, or Providence, aquifer. It is composed of sand, shell, and gravel layers and some kaolin deposits of commercial and non-commercial value. The kaolin clays can be seen in small road cuts along Interstate 75 in Houston and Peach Counties. Sediments of the Cretaceous aquifer represent ancient shoreline deposits laid down approximately 80-100 million years ago when the Fall Line was the coastline of Georgia. The Cretaceous aquifer thus crops out along and near the Fall Line. It is a very productive aquifer providing abundant water supplies for agricultural and municipal users in the northern part of the Lower FRB. Above the Cretaceous aquifer is the Clayton aquifer, which is a sandy formation towards the north but a limestone towards the south. The Clayton is a highly productive aquifer in the northwestern part of the lower FRB; however, unlike the other FRB aquifers it has a very small outcrop area and thus receives very little recharge from rainfall. The combination of the Clayton’s high productivity and its small recharge area caused water levels in the Clayton to decline precipitously. In 1992, EPD imposed a permanent moratorium on new withdrawals from the Clayton. Overlying the Clayton is the Claiborne aquifer. The Claiborne is typically a sandy aquifer like the Cretaceous aquifer, but it also contains more fine-grained sediment and consequently is not as productive as the Cretaceous in the northern part of the lower FRB. However, the Claiborne aquifer is very productive in parts of Sumter, Dooly, Lee, and Dougherty counties and is relied upon heavily in these areas for agricultural, industrial, and municipal water supply. The Claiborne has a much larger outcrop area than the Clayton, and thus recharges annually if rainfall is sufficient. The Floridan aquifer lies above the Claiborne aquifer, and is one of the most productive aquifers in the world. It consists of a highly fossiliferous limestone that has developed an extremely high level of secondary porosity and permeability. In other words, groundwater seeping through the Flint River Basin: Technical Summary of Hydrogeology, Farm Water Use, and Ecology Page 3 limestone has widened natural fractures, bedding planes, fossil moulds, etc., and created an extremely permeable layer. Furthermore, the Floridan aquifer is exposed over a very wide area…most of the Dougherty Plain…and thus receives complete recharge every year if there is sufficient rainfall. Because the limestone is exposed at the surface, the streams and rivers in that area have cut down into the limestone. Therefore, the Floridan aquifer is in direct hydraulic connection with much of the surface water drainage network in the lower Flint River Basin. The Flint River and its tributaries receive hundreds of millions of gallons of water every day from the limestone. This is evident from the numerous springs along the stream channels in the Dougherty Plain. from Lake Blackshear to Lake Seminole, springs like Radium Springs and others provide the Flint River with large amounts of water. In fact, more than 20 major springs have been mapped by the Georgia DNR in the river section between Albany and Bainbridge. Furthermore, large volumes of groundwater seep directly through streambeds where bare limestone is exposed. The flow of water between the streams and the aquifer is referred to as “stream-aquifer flux”. The amount of water that seeps or flows into the streams is controlled by the hydraulic pressure differential between the stream and the Floridan aquifer. As long as the head in the aquifer is higher than that of the streams, water will flow from the aquifer into the streams. This is called an effluent stream condition. However, if head in the aquifer drops below that of the stream, then water will seep or flow from the stream into the aquifer. This is called an influent stream condition. If a stream is an effluent stream, then the rate and volume of water entering the stream will depend on the head in the aquifer and the size of the spring opening. The greater the hydraulic differential, the more water will flow into the streams. It is like holding a water-filled funnel and siphon with your thumb over the end of the siphon tube. The higher the funnel is held, the greater the flow out of the tube when your thumb is released. Not all the water that recharges the Floridan aquifer is released into the streams. The streams only cut into the upper few tens of feet of the aquifer (even though they receive most of the water that the aquifer discharges naturally).